The present invention relates in general to friction clutches and, more particularly, the invention relates to an apparatus which determines when such friction clutch is being operated in a manner which will cause overheating and preventing such manner of operation from continuing beyond a predetermined time.
As is known in the art, friction clutches are used to make an adherence-actuated connection between two moving parts or to separate them. During the normal clutch process, the individual clutch parts can rub against one another for a rather prolonged period of time. This situation occurs particularly on motor vehicle clutches during the start-up and shifting processes when the speeds of the motor and driveshaft must be equalized with one another. As a general rule, motor vehicle clutches are designed so that they can withstand these processes without serious damage. But situations many times occur, particularly in a motor vehicle, in which clutch slip does occur over a relatively long period of time. For example, with a manually-operated clutch, this can occur on a hill when the driver uses the clutch to hold the vehicle on the hill by slipping the clutch and partly depressing the gas pedal. It can also occur when a vehicle equipped with an automatic clutch is kept on a hill for a long period of time with the gas pedal partly depressed. On vehicles equipped with a manually-operated clutch, the driver is generally aware of the danger of "riding" the clutch, and after a short period of time, he will then switch to holding the vehicle on the hill with the brake. On vehicles equipped with automatic clutches, however, he is generally not so aware of the danger of such a condition since he only needs to give the engine a small amount of gas to hold the vehicle on the hill.
One attempt to prevent situations where overheating occurs, it has been proposed in DE-OS No. 3206593 that the rotational slip between the input and output shaft be determined and that a signal be determined from this quantity which, after a predetermined time, is sent to the driver. This apparatus operates very imprecisely, however, because the transmitted moment of friction contributes to the heating of the coupling in an altogether different manner, which is not taken into account in the apparatus described by DE-OS No. 3206593, so that protection against overheating is not ensured under all operating conditions that may be encountered in normal operation of a vehicle.